The major goal of this project is to test the idea that modulation of autophagy within the hypoxic tumor microenvironment (TME) can impact circulating tumor cell (CTC) survival, metastatic potential, and therapeutic response. The TME is a complex breeding ground for selection of aggressive tumor cells with an advantage of survival or metastatic potential and confers resistance to cytotoxic as well as targeted cancer therapy. The process of tumor metastasis consists of multiple steps, including tumor cell dissemination from the primary tumor site into the vasculature or lymphatic circulation (intravasation), survival during circulation, extravasation into the secondary site, and initiation and colonization at the target organ site. Tumor cells must successfully complete each step to give rise to clinically detectable metastatic disease. It is generally accepted that autophagy is essential for tumor cell survival under conditions of nutrient or oxygen deprivation, the hallmarks of the TME. However, the relative contribution of autophagy within the TME to the pro-survival, metastasis- prone, and therapy-resistant phenotypes of CTCs is similarly unknown. There is therefore a great opportunity to unravel this biology and shed light on better therapeutic designs as well as therapy monitoring. We hypothesize that hypoxia-induced autophagy within the TME contributes to CTC survival, tumor metastasis, and chemoresistance. To test this hypothesis, we propose the following Specific Aims: 1) To determine the importance of hypoxia-regulated autophagy in tumor growth and metastasis in xenograft mouse models; 2) To evaluate autophagy and apoptosis in xenograft tumors and CTCs; 3) To investigate the effect of hypoxia- induced autophagy on therapeutic response as detected and monitored in CTCs. PUBLIC HEALTH RELEVANCE: Successful implementation of this research will not only offer a better understanding of the biological functions of hypoxia-induced autophagy within the TME in tumor cell survival, metastatic potential, and therapeutic resistance, but will also aid in the effort to develop an invaluable tool to monitor molecular events and biomarkers in CTCs for guiding diagnosis, prognosis and treatment of cancer.